RULE for take is too strict

main = print (map (const 'x') (take 1 (undefined:undefined)))

In ghci, or with ghc -O0, this produces "x". With ghc -O, this produces Prelude.undefined. Kirsten Chevalier found that the culprit is the RULE:

"take"     [~1] forall n xs . take n xs 
    = case n of I# n# -> build (\c nil -> foldr (takeFB c nil) 
                                                (takeConst nil) xs n#)

This rule makes take strict in xs, but it should be lazy when n=0.

Trac metadata

Trac field	Value
Version	6.6
Type	Bug
TypeOfFailure	OtherFailure
Priority	normal
Resolution	Unresolved
Component	libraries/base
Test case
Differential revisions
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Operating system	Unknown
Architecture	Unknown